THANKS A LOT FOR THE QUICK RESPONSE AND GREAT ASSIST

Try this code insert into "Tool change G-code":

Ah, didnt consider that use case.  But heres another suggestion.  Create some gcode that is the preheat temperatures you want followed by the beeps etc and put it on the sd card.  Then instead of going to the menu and preheat for xxx just print from sd card and load the file.  That will start the preheat and beep when temp reached as it 'prints',  Just dont have the end gcode for the cooling down.  There's nothing saying that a gcode file actually has to extrude anything.

You could create a few for different files for other temps and or preheat bed or nozzle or both etc.  btw my normal start up gcode has a preheat to 165 degrees nozzle with 65 on the bed while it waits for the pinda temp to stabilise before it then goes on to normal printing.  Point being that if you preheat to 165 the nozzle doesnt leak and its quick for the increase to printing temp so its a good preheat value to have.  If you are preheating to say 215 for pla and then leaving it at that temp you risk cooking your filament which can possibly increase your chance of a clog.

Erm that doesnt need any software changes to implement.  With Prusa SLicer in expert mode go to Printer Settings > Custom G-Code and then look for the start gcode entry box.

If you are running anything remotely standard prusa printer wise you will see the lines for heating the bed and extruder to temp and waiting till they reach temp, something like the following-
M104 S ; set extruder temp
M140 S ; set bed temp
M190 S ; wait for bed temp
M109 S ; wait for extruder temp

You can then add an M300 command like M300 S294 P200 after that which will play a tone, or add several with different parameters and play a tune if you like.

Thanks for the suggestion, but afraid it won't help me, since I'm preheating right after powering the printer on, before slicing and loading any gcode :)

If you are running anything remotely standard prusa printer wise you will see the lines for heating the bed and extruder to temp and waiting till they reach temp, something like the following-
M104 S ; set extruder temp
M140 S ; set bed temp
M190 S ; wait for bed temp
M109 S ; wait for extruder temp

You can then add an M300 command like M300 S294 P200 after that which will play a tone, or add several with different parameters and play a tune if you like.